This invention relates to the field of semiconductor integrated devices, and particularly relates to a process of surface passivation for improving adhesion between a nickel silicide layer and a silicon nitride layer.
The performance of a silicon integrated circuit is related to its component size. Small components enable more integrated functionality and faster operation. One technical challenge to the engineers in their endeavor to reduce the silicon integrated circuit component size is to maintain the conductance of the conductive lines that hook up the circuit components. One type of conductive line currently used in the silicon integrated circuit industry is refractory metal silicide clad silicon. For example, cobalt silicide clad silicon is the material of choice in making MOS transistor gate electrodes and the source and drain region of high-performance logic circuits.
Cobalt silicide clad silicon served its designed purpose well. But cobalt silicide process encounters problems when polysilicon line becomes narrower than 50 nanometers. One problem is that cobalt silicide requires a temperature of about 700xc2x0 C. to form, which over taxes the thermal budget of total process. Another problem is at 700xc2x0 C., grains in the polysilicon conglomerate, which reduces the polysilicon volume and causes the formation of voids in the polysilicon lines. Depending on the width of the polysilicon line and the density and the size of the voids, the conductance of the polysilicon line may vary substantially.
To overcome the shortcomings of cobalt silicide, engineers turn to nickel silicide. Nickel reacts with silicon and forms nickel silicide at a temperature below 300xc2x0 C. We have demonstrated that between 260xc2x0 C. and 310xc2x0 C., we can form nickel silicide alloy that is rich in nickel and has stable sheet resistance with good uniformity. The resistivity drops further when the alloy is treated at between 400xc2x0 C. to 550xc2x0 C. and the alloy converts substantially to nickel mono-silicide. The lowering of process temperature to about 500xc2x0 C. effectively eliminates or substantially reduces the problem of polysilicon grain conglomeration and substantially conserves the thermal budget of the total process.
There are problems, however, that keep the nickel silicide clad polysilicon process from widely implemented. One known problem is that the insulation layer, usually a silicon nitride film, which forms on the surface of the nickel suicide, has a tendency to blister or peel off from the silicide surface. This poor adhesion of this insulation layer causes undesirable excessive power consumption and reliability problem.
Engineers in semiconductor equipment manufacturing companies and integrated circuit manufacturing companies have been trying to solve the adhesion problem without success. The present invention effectively eliminates or substantially reduces this problem.
We have determined that the root cause of the poor adhesion between the nickel silicide and the silicon nitride is the presence of a silicon rich interface film. The present invention eliminates or substantially reduces the adhesion problem by preventing such film from formation.
In the known art, a thin silicon nitride layer is usually provided in combination with a thicker silicon dioxide film for insulating the conductive nickel silicide material electrically from other conductive materials. It is usually formed in a plasma reactor with a plasma enhanced chemical vapor deposition (PECVD) process.
The environment of the reactor is a gaseous mixture comprises ammonia and silane. Energy in the form of radio frequency signal activates molecules of silane and ammonia in the plasma and produces silicon and nitrogen species. Silicon and nitrogen species react and form silicon nitride on the surface of the semiconductor substrate.
In the presence of nickel silicide, however, silane decomposes and produces silicon without the aid the radio frequency signal. Without an abundance of activated nitrogen to form the desired silicon nitride, the silicon species precipitates on the substrate surface and forms a silicon rich film that is the cause of the adhesion problem.
The present invention solves this problem by preventing the formation of this silicon rich film. In one embodiment of the present invention, ammonia is first activated by the radio frequency signal in the absence of silicon carrying gas. The nitrogen species from the activated ammonia passivates the nickel silicide surface. Subsequently, silane or other silicon carrying gas may be mixed in the reactor to form silicon nitride.
In another embodiment of the present invention, nitrogen gas is first activated by the radio frequency signal in the absence of silicon carrying gas. The nitrogen species from the activated nitrogen gas passivates the nickel silicide surface. Subsequently, silane or other silicon carrying gas may be mixed in the reactor to form silicon nitride.
In yet another embodiment of the present invention, the semiconductor substrate is deposited with a film of titanium nitride or other transition metal nitride. The nitrogen species in the nitride film reacts with the nickel silicide at an elevated temperature around 500xc2x0 C. to passivate the nickel silicide surface. Once the surface is adequately passivated and the residual metal nitride layer is removed, silicon nitride film formation may proceed.
In yet another embodiment of the present invention, a physical sputtering process deposits the silicon nitride film on the semiconductor substrate where the target comprises silicon nitride material.